It has previously been proposed to adjust the purity of color in a TV receiver by the "red ball method", which, briefly, is carried out this way:
The apparatus is located in North-South direction and a signal is applied thereto which should provide a red surface display. By shifting the deflection system backwardly and forwardly, it is possible to place a raster of pure red color derived by the cathode beam. The thus generated pure-red color is then shifted to occur at the center position of the cathode ray tube (CRT) by rotating the color-purity magnets. After centering, and upon considering the static convergence, the deflection system is so shifted or moved that the red surface will be uniform over the entire surface of the CRT. The blue and green zones are also checked.
The precise position of the deflection system on the neck of the CRT can be located by checking the screen with a microscope. The deflection system is so positioned that the beam will impinge on the appropriate phosphor points also in the corners of the CRT. The deflection system is then secured to the neck of the CRT, for example by clamping, and is adhesively connected and sealed by rubber wedges and the like to the neck of the CRT.
Fine adjustment of the CRT with a self-converging sytem after locating the receiver in a specific place usually is not provided. Likewise, precisely adjusted deflection systems having convergence circuits are usually not readjusted after locating the receiver in a specific place.
Particularly precisely adjusted receivers, which were worked on by qualified, highly skilled personnel, usually have excellent convergence and beam impingement location. It is desirable to avoid readjustment or recalibration after the apparatus is placed in a specific user's location; usually no skilled personnel and monitoring equipment is available at such locations.
It is possible that, when a receiver is located at a desired random location, stray or foreign fields may interfere with color purity. This may occur, for example, due to substantial change in the magnetic field of the earth. For color purity, the color purity magnets will then have to be readjusted. The position of the deflection system may be retained.
If the above referred-to and standard sequence of adjustment is followed, finding the exact impingement point of the beam requires loosening and shifting of the deflection system. Thus, a position which previously was found and carefully and precisely adjusted is lost again, and the overall calibration for color purity at a specific location must be done anew. Frequently, the conditions under which readjustment is to be made are poor.
Complete correction and adjustment of color purity over the entire surface of the CRT involves adjustment, checking, readjustment, rechecking, over and over again, straining the patience of operating personnel and resulting, hardly ever, in perfectly adjusted receivers.